In recent years, quietude has been increasingly required of the recording disk drive unit including a hard disk with the increase in the rotational speed. For this reason, the fluid dynamic bearing has come to be widely used for the hard disk drive. The fluid dynamic bearing is configured of a fixed member and a rotational member. The fixed member and the rotational member are arranged in opposed relation, without mutual contact, to each other through a very small bearing gap. A lubricating fluid is filled in the bearing gap and the rotational member is rotatably supported on the fixed member by the dynamic pressure generated in the lubricating fluid.
The bearing gap between the component members of the fluid dynamic bearing is very narrow. In assembling the fluid dynamic bearing, therefore, a slight deformation of any member constituting the bearing would lead to the failure to achieve the intended performance or make a defective product. Therefore, the welding process, rather than pressure fitting or fitting, has come to be employed for assembling the fluid dynamic bearing.
The fixing process by welding causes a lesser deformation of the component members as a whole than the fixing process by pressure fitting or fitting. Further, as compared with the fixing process by bonding, the welding requires no setting time and therefore is higher in productivity. During the fixing process by bonding, a gas is emanated out of the organic adhesive solvent when the adhesive is set. The welding, on the other hand, generates no such gas, and can prevent the surface of the component members of the fluid dynamic bearing and the interior of the disk drive from being contaminated.
In the case where the welding process is used, however, the material is required to be heated to the melting point at the weld zone. A smaller fluid dynamic bearing is conspicuously affected by this heat. Especially, the lubricating oil used as a lubricating fluid generally cannot stand the high welding temperature. Further, the component members of the fluid dynamic bearing develop an uneven deformation by local heat. Specifically, the radial dynamic bearing unit for supporting the radial load has so small a bearing gap that special care must be taken of the effect of heat. In view of these problems, the lubricating oil is injected into the fluid dynamic bearing after being completely assembled.
To assure the assembly convenience of the fluid dynamic bearing, on the other hand, it is desired to inject the lubricating fluid before the welding process. A fluid dynamic bearing is available having such a structure that the interface between the lubricating oil and the air external to the fluid dynamic bearing is protected by a cover member to prevent the evaporation of the lubricating fluid or prevent the gasified oil from being discharged out of the fluid dynamic bearing. In the case where the adhesive and the lubricating oil come into contact with each other, the performance of the adhesive and the lubricating oil may be deteriorated by the chemical reaction. Also, in the case where the cover member has the function as a stopper of the rotational member, a heavy load may be imposed on the cover member due to an impact which may be exerted on the fluid dynamic bearing. Therefore, the cover member is desirably fixed by welding. In the case where the lubricating oil is injected after mounting the cover member, however, the lubricating oil flies out of the bearing and is required to be wiped off. This process brings about a reduced workability and an increased cost.
As described above, the conventional manufacturing method in which the parts are welded without adversely affecting the productivity is not existent.
A first object of this invention is to provide a method of manufacturing the fluid dynamic bearing in which the cover member is fixed by welding after the fluid dynamic bearing holds the lubricating oil.
A second object of the invention is to provide an inexpensive, reliable spindle motor.
A third object of the invention is to provide an inexpensive, reliable recording disk drive unit.